Add extension to allow defining image width in markdown

This allows setting image width in markdown, for example:

![alt text](image.png){ width=50% }

Which is very useful to make the image sizes dynamic. Could be useful in future group assignments.

mentioned in commit 1e203d6f

merged

Sorry, I should have tested this first but it doesn't seem to work with this theme template. It works on my site with the Material for Mkdocs theme.

Not sure if this can be fixed

If you want I can do a rollback. This theme is not using the Material, but it is a theme I built based on Material Mkdocs (https://gitlab.fabcloud.org/ivanmilara/fabacademy_fab_theme). I could have a look why this is not working, but not in the following days. Feel free to debug the problem yourself.

It shouldn't hurt having that there, maybe I'll too look in to it later.